update thumbnail, allow user to choose output format

[kouz75]

Add output format support to thumbnail URLs

• Add optional format parameter to ThumbnailHelper, ImageExtension, and the thumbnail() Twig function
• Encode the output format as a filename suffix in the URL (e.g. image.jpg.avif) instead of in the param string
• Parse the format suffix in ImageController to set Glide's fm parameter and resolve the correct source file
• Backward compatible: URLs without a format suffix continue to work unchanged